α-Sulfonyl Carbanions: Combined Kinetic, Thermodynamic, and NMR

9496

J. Am. Chem. Soc. 1998, 120, 9496-9503

R-Sulfonyl Carbanions: Combined Kinetic, Thermodynamic, and NMR Approaches for the Study of the Ionization of Benzyltriflones in Me2SO and H2O-Me2SO Mixtures F. Terrier,* E. Kizilian, R. Goumont, N. Faucher, and C. Wakselman Contribution from the Laboratoire SIRCOB, EP CNRS 102, Department of Chemistry, UniVersity of Versailles, 45, AVenue des Etats-Unis, 78035 Versailles Cedex, France ReceiVed February 23, 1998. ReVised Manuscript ReceiVed July 1, 1998

Abstract: The ionization of a series of benzyltriflones ArCH2SO2CF3 (Ar ) C6H5, 4-CNC6H4, 4-NO2C6H4, 2,4-(NO2)2C6H3, 2,4,6-(NO2)3C6H2) has been studied kinetically and thermodynamically as well as by 1H, 13C, and 19F NMR in H O-Me SO mixtures and/or Me SO. The findings that the acidity of the triflones is 2 2 2 enhanced on going from hydroxylic solvents to Me2SO and that their deprotonation is associated with high Marcus intrinsic reactivities are two major results indicating the presence of a large polarizable charge density at CR of the carbanionic species. Comparison of the results obtained with those for related arylnitromethane and arylacetonitrile compounds strongly supports this conclusion. NMR data add to the evidence that polarization rather than d-p π-resonance or negative hyperconjugation is the dominant factor in accounting for the capability of the strongly electron withdrawing SO2CF3 group to stabilize negative charge. Altogether, our results lead to a structural picture of the triflone carbanions in solution which does not totally fit the conclusions reached on the basis of solid-state or computed gas-phase studies.

Carbanions that are stabilized by carbonyl, cyano, nitro, and sulfonyl or sulfoxide groups have received wide use in synthesis, and some of them are important biochemical intermediates.1 Considerable evidence has been accumulated that carbanions R to nitro and carbonyl groups derive much of their stability by rehybridization from sp3 to sp2, thus allowing delocalization of charge to the more electronegative atoms.1a-c In contrast, it has been less clear how conjugative effects are important in determining the stability of carbanions adjacent to cyano or sulfur groups.1d-h Recently, a number of studies have appeared which convincingly support the view that the polar effect of the cyano group is the main factor governing the stability of R-cyanocarbanions.2-4 The finding that nitriles are carbon acids exhibiting much higher intrinsic Marcus reactivities (log k0 ≈ 7) than carbonyl (log k0 ≈ 3-4) or nitro (log k0 < 2) compounds was very revealing in this regard.4 * Corresponding author. Telephone: 00-33-1-39-25-44-50. Fax: 00-331-39-25-44-52. E-mail: [email protected]. (1) (a) Trost, B. M., Ed. ComprehensiVe Organic Synthesis, Vol. 3: Carbon-Carbon σ-Βond Formation; Pergamon Press: Oxford, U.K., 1991. (b) Richard, J. P. In The Chemistry of Enols; Rappoport, Z., Ed.; Wiley: New York, 1990. (c) Nielsen, A. T. In Nitrones, Nitronates and Nitroxides; Breuer, E., Aurich, H. G., Nielsen, A. T., Eds.; Wiley: New York, 1989. (d) Simpkins, N. S. In Sulphones in Organic Synthesis; Pergamon Press: Oxford, U.K., 1993. (e) Oae, S.; Uchida, Y. In The Chemistry of Sulphones and Sulphoxides; Patai, S., Rappoport, Z., Stirling, C. J. M., Eds.; Wiley: New York, 1988; Chapter 12. (f) Buncel, E. In Carbanions: Mechanistic and Isotopic Aspects; Eaborn, C., Chapman, N. B., Eds.; Elsevier: Amsterdam, 1993; Chapter 2, pp 60-64. (g) Hoz, S. Ibid. 1993; p 1. (h) Boche, G. Angew. Chem., Int. Ed. Engl. 1989, 28, 277. (2) (a) Bradamante, S.; Pagani, G. A. J. Chem. Soc., Perkin Trans. 2 1986, 1035. (b) Abbalto, A.; Bradamante, S.; Pagani, G. A. J. Org. Chem. 1993, 58, 449. (3) Wiberg, K. B.; Castejon, H. J. Org. Chem. 1995, 60, 6327. (4) (a) Bernasconi, C. F. AdV. Phys. Org. Chem. 1992, 27, 119. (b) Bernasconi, C. F.; Stronach, M. W. J. Am. Chem. Soc. 1990, 112, 8448.

The situation for R-sulfur-stabilized carbanions, especially R-sulfonyl carbanions has long been a subject of much debate.1d-h The enhanced stability of R-sulfonyl carbanions has been first explained in terms of a substantial d-p π-bonding between the carbanion lone pair and sulfur 3d orbitals, as depicted in 1.5-8 However, this resonance effect has been questioned by theoretical studies and carbanion stabilization by simple charge polarization effects has been favored, i.e., 2.7,9-15 In more recent years, negative hyperconjugation has also been suggested as an important contributing mechanism, i.e., 3, especially in fluorinated carbanions such as triflone carbanions (R ) CF3).7,14,16-19

In as much as the intrinsic reactivity (in the Marcus sense) of a carbon acid is closely related to the extent of the structural and solvational reorganization that is needed to form the conjugate carbanion, a measure of this parameter for some R-sulfonyl derivatives should help clarify the mode of action (5) Bordwell, F. G.; Vanier, N. R.; Matthews, W. S.; Hendrickson, J. B.; Skipper, P. L. J. Am. Chem. Soc. 1975, 97, 7160. (6) Oae, S.; Tagaki, W.; Ohno, A. Tetrahedron 1964, 20, 417. (7) (a) Wolfe, S.; Stolow, A.; Lajohn, L. A. Tetrahedron Lett. 1983, 24, 4071. (b) Wolfe, S.; Lajohn, L. A.; Bernardi, F.; Mangini, A.; Tonachini, G. Ibid. 1983, 24, 3789. (c) Wolfe, S.; Bernardi, F.; Csizmaldia, I. G.; Mangini, A. In Organic Sulfur Chemistry; Elsevier: Amsterdam, 1985; pp 133-190. (8) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry, 3rd ed.; Plenum Press: New York, 1990; Part A, p 408.

S0002-7863(98)00607-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/09/1998

R-Sulfonyl Carbanions of the SO2R group.4,20,21 Thus, if the charge delocalization mechanisms, i.e., 1 and 3, and not the polarizability or inductive effects are important in determining the stability of conjugate carbanions, sulfones should exhibit relatively low intrinsic reactivities, comparable at least with those reported for carbon acid structures giving rise to moderately resonance-stabilized carbanions.4 In this paper, we report a structural, thermodynamic, and kinetic study of the ionization of the various benzyltriflones 4a-e to give the conjugate carbanions C-4a-e in Me2SO and some H2O-Me2SO mixtures according to eq 1. Altogether,

J. Am. Chem. Soc., Vol. 120, No. 37, 1998 9497 Table 1. Effect of Ring Substituents on Chemical Shifts (∆δHR, ∆δCR) and Coupling Constants (∆1JCRHR) upon Ionization of Benzyltriflones 4a-e and Comparison with Picryl Ketones 8a,ba

b

compd

δHR

4a C-4a 4b C-4b 4c C-4c 4d C-4d 4e C-4e 8ab C-8ab 8bb C-8bb

5.25 3.38 5.46 3.69 5.53 4.07 5.87 5.78 5.83 5.82 4.43 6.55 5.07 7.26

∆δHR -1.87 -1.77 -1.46 -0.09 -0.01 2.12 2.19

δCR 54.80 55.02 54.31 60.20 53.74 65.72 51.99 72.25 48.96 73.46 42.59 99.89 39.11 95.32

∆δCR 0.22 5.89 11.98 20.26 24.43 57.3 56.21

1J

CRHR

142.77 163.89 143.41 167.42 143.15 168.02 145.33 176.98 146.60 172.26 132.3 159.6 132.3 158.9

∆1JCRHR 21.12 24.01 24.87 31.65 25.66 27.3 26.6

a Solvent Me2SO-d6; δ in ppm relative to internal SiMe4; J in hertz. References 21 and 41d.

Table 2. pKa Values for Benzyltriflones 4a-e and Related Carbon Acids in Me2SO and Various H2O-Me2SO Mixtures compd H2O 30% Me2SO 50% Me2SO 70% Me2SO

our results favor the idea that charge polarization effects are by far the most important mechanism contributing to the stabilization of C-4a-e, including in the picryl system. Implications of this conclusion with regard to the structures of the carbanions in solution are considered by comparison with solid-state and gas-phase data.9,14-19,22 Results Structural Studies. Addition of dilute tetramethylammonium hydroxide to ∼4 × 10-5 M solutions of 4a-e in H2OMe2SO mixtures containing 30%, 50%, and 70% Me2SO by volume results in the reversible and complete formation of species exhibiting intense absorption maxima at the following wavelengths: 286 nm (C-4a); 343 nm (C-4b); 473 nm (C-4c); 429 nm (C-4d); 437 nm (C-4e). Unambiguous evidence that base addition to 4a-e actually affords the carbanions C-4a-e was obtained by carrying out a 1H, 13C, and 19F NMR study of the ionization reactions in Me2SO. Only the data required for (9) (a) Bors, D. A.; Streitwieser, A., Jr. J. Am. Chem. Soc. 1986, 108, 1397. (b) Speers, P.; Laidig, K. E.; Streitwieser, A. J., Jr. Ibid. 1994, 116, 9257. (10) Koch, R.; Anders, E. J. Org. Chem. 1994, 59, 4529. (11) Patterson, C. H.; Messmer, R. P. J. Am. Chem. Soc. 1989, 111, 8059. (12) Lehn, J. M.; Wipff, G. J. Am. Chem. Soc. 1976, 98, 7498. (13) Bernardi, F.; Csizmaldia, I. G.; Mangini, A.; Schlegel, H. B.; Wangbo, M. H.; Wolfe, S. J. Am. Chem. Soc. 1975, 97, 2209. (14) Schleyer, P. v. R.; Clark, T.; Kos, A. J.; Spitznagel, G. W.; Rohde, C.; Arad, D.; Houk, K. N.; Rondan, N. G. J. Am. Chem. Soc. 1984, 106, 6467. (15) Larson, J. R.; Epiotis, N. D. J. Am. Chem. Soc. 1981, 103, 410. (16) Schleyer, P. v. R.; Kos, A. J. Tetrahedron, 1983, 39, 1141. (17) Wiberg, K. B.; Castejon, H. J. Am. Chem. Soc. 1994, 116, 10489 and references therein. (18) (a) Gais, H. J.; Hellmann, G.; Gu¨nther, H.; Lopez, F.; Lindner, H. J.; Braun, S. Angew. Chem., Int. Ed. Engl. 1989, 28, 1025. (b) Gais, H. J.; Mu¨ller, J.; Vollhardt, J.; Lindner, H. J. J. Am. Chem. Soc. 1991, 113, 4002. (19) Raabe, G.; Gais, H. J.; Fleischhauer, J. J. Am. Chem. Soc. 1996, 118, 4622. (20) (a) Bernasconi, C. F. Pure Appl. Chem. 1982, 54, 2335. (b) Bernasconi, C. F. Tetrahedron 1985, 41, 3219. (c) Bernasconi, C. F. Acc. Chem. Res. 1987, 20, 301. (21) (a) Moutiers, G.; El Fahid, B.; Goumont, R.; Chatrousse, A. P.; Terrier, F. J. Org. Chem. 1996, 61, 1978. (b) Moutiers, G.; El Fahid, B.; Collot, A. G.; Terrier, F. J. Chem. Soc., Perkin Trans. 2 1996, 49.

4aa 4ba 4ca 4da 4ea 5a 5b 5c 6b 6c 6d 6e 6f

12.82 11.75 7.98 6.87 6.77b 13.45f 9.91g

15.10 12.00 10.57 6.94 6.16 7.93b

11.02 9.55 5.61 4.40 8.53b

6.02,e 6.00 12.62a 8.06g 12.17g 12.02g 10.71g

Me2SO 14.62 10.70 9.46 5.05 4.47 12.32,b 12.20c 9.31d 8.62d 12.36,a 12.30i 6.00g,h 11.15g,h 11.03g,h 9.24g,h

a This work at 25 °C. b Reference 28 at 20 °C. c Reference 30 at 25 °C. d Reference 31 at 25 °C. e Reference 29 at 25 °C. f References 3234 at 25 °C. g Reference 36 at 20 °C. h 90% Me2SO-10% H2O. i Bordwell’s value quoted by Walters in ref 62c.

discussion are given in Table 1. All chemical shift and coupling constant data are summarized in Supporting Information Tables S1-S3.23 Equilibrium Measurements. Using appropriate buffers values of (carboxylic acids, phenols, and amines), the pKCH a 4a-e were first determined in various H2O-Me2SO mixtures from observed absorbance variations at λmax of C-4a-e obtained at equilibrium as a function of pH. These variations described regular acid-base equilibrations, as evidenced by Figure S1, which shows that excellent straight lines with unit slopes were obtained on plotting the log values of the ratio of the concentrations of ionized to nonionized triflones as a function of pH (eq values thus obtained at 25 °C and constant 2).23 The pKCH a ionic strength of 0.5 M (NMe4Cl) are summarized in Table 2.

[C-2] ) pH - pKCH log a [2]

(2)

The pKCH values of 4a-e in pure Me2SO solution were a measured as follows. In the case of 4a, the ionization was found to occur in the same pH range as that for 2,4-dinitroaniline (pKa (22) (a) Gais, H. J.; Vollhardt, J.; Lindner, H. J. Angew. Chem., Int. Ed. Engl. 1986, 25, 939. (b) Gais, H. J.; Hellmann, G.; Lindner, H. J. Ibid. 1990, 29, 100. (c) Boche, G.; Marsch, M.; Harms, K.; Sheldrick, G. M. Ibid. 1985, 24, 573. (d) Hollstein, W.; Harms, K.; Marsch, M.; Boche, G. Ibid. 1988, 27, 846. (23) See paragraph concerning Supporting Information at the end of this paper.

9498 J. Am. Chem. Soc., Vol. 120, No. 37, 1998 ) 15.10) and 4,4′-dinitrodiphenylmethane (pKa ) 15.90).24 Since these two compounds have conjugate bases exhibiting an intense visible absorption at λmax ) 542 nm ( ) 17 320 mol L-1 cm-1) and λmax ) 782 nm ( ) 9.7 × 104 mol L-1 cm-1), respectively, they were conveniently used as indicators to measure the pH of various buffer solutions made up from 4a (Figure S2).23 Then, the pKCH a value of 4a was readily obtained from eq 2. In the case of the other triflones (4b-e), the ionization occurred in pH ranges covered by various amine buffers.25-27 Accordingly, the pKCH values of these triflones were detera mined according to the same procedure as that used in Η2OMe2SO mixtures. The results are also given in Table 2, together with similar data for the arylnitromethanes 5a-c28-31 and the arylacetonitriles 6a-f.32-36

Terrier et al. Table 3. Rate Constants for the Ionization of 2,4-Dinitrobenzyltriflone 4d (pKCH a ) 6.94) in 50% H2O-50% Me2SO (v/v)a buffer (basic species B) aminoacetonitrile glycine ethyl ester glycinamide 2-methoxyethylamine

no.

pKBH a

kBp , M-1 s-1

BH k-p , M-1 s-1

1 2 3 4

5.26 7.24 8.01 9.11

10b 6050 9350 30600

52000 2900 795b 206b

I ) 0.5 M (NMe4Cl); T ) 25 °C; experimental error in the rate CH constants: (5%; in pKBH a and pKa : (0.05 pK units; in the rate constants calculated from eq 5: (15%; pKBH values from ref 37. a BH b Calculated from k-p or kBp via eq 5. a

was associated with the interconversion of 4d and C-4d. The observed pseudo-first-order rate constants, kobsd, measured in the various buffer systems employed are summarized in Table S4.23 2O + kB[B] + kOH[OH-] kH p p p

4a-e {\ } C-4a-e H2O H BH + k-p[H ] + k-p [BH] + k-p

(

B kobsd ) kBp [B] + kBH -p [BH] ) kp +

Kinetic Measurements. We have found that rates of equilibration of 4a-e with their conjugate bases C-4a-e according to eq 3 were too high to be accessible by stoppedflow spectrophotometry under most reasonable experimental conditions in buffer as well as NMe4OH solutions in Η2OMe2SO mixtures. Only in the case of the 2,4-dinitrobenzyltriflone 4d could some reliable kinetic measurements be carried out at 25 °C and constant ionic strength of 0.5 M (NMe4Cl) in 50% H2O-50% Me2SO (v/v). Buffers made up from primary CH amines with pKBH a values close to the pKa value of 4d were used, i.e., glycine ethyl ester, glycinamide, 2-methoxyethylamine, and aminoacetonitrile.37 Experiments were performed under pseudo-first-order conditions with a large excess of the buffer reagents over the triflone concentration (≈5 × 10-5 M). Depending upon the pH studied, equilibrium 3 was approached either from the reactant side (pΗ g pKCH a ) or the product side (pH e pKCH ), but in all instances, only one relaxation process a (24) Bordwell, F. G.; Algrim, D. J. J. Am. Chem. Soc. 1988, 110, 2964. (25) Crampton, M. R.; Robotham, I. A. J. Chem. Res. (S) 1997, 22. (26) Kolthoff, Z. M.; Chantooni, M. K., Jr.; Bhowmick, S. J. Am. Chem. Soc. 1968, 90, 23. (27) Courtot-Coupez, J.; Le Demezet, M. Bull. Soc. Chim. Fr. 1969, 1033. (28) (a) Bernasconi, C. F.; Kliner, D. A. V.; Mullin, A. S.; Ni, J. X. J. Org. Chem. 1988, 53, 3342. (b) Bernasconi, C. F.; Ni, J. X. J. Am. Chem. Soc. 1993, 115, 5060. (29) Moutiers, G.; Thuet, V.; Terrier, F. J. Chem. Soc., Perkin Trans. 2 1997, 1479. (30) (a) Bordwell, F. G.; Satish, A. V. J. Am. Chem. Soc. 1994, 116, 8885. (b) Bordwell, F. G.; Zhao, Y. J. Org. Chem. 1995, 60, 6348. (31) Keeffe, J. R.; Morey, J.; Palmer, C. A.; Lee, J. C. J. Am. Chem. Soc. 1979, 101, 1295. (32) (a) Schaal, R. J. Chim. Phys. 1955, 52, 784, 796. (b) Masure, F.; Schaal, R.; Souchay, P. Bull. Soc. Chim. Fr. 1956, 1143. (c) Schaal, R.; Favier, P. Ibid. 1959, 2011. (33) Deno, N. C. J. Am. Chem. Soc. 1952, 74, 2039. (34) Langford, C. H.; Burwell, R. L., Jr. J. Am. Chem. Soc. 1960, 82, 1503. (35) Bernasconi, C. F.; Hibdon, S. A. J. Am. Chem. Soc. 1983, 105, 4343. (36) Bernasconi, C. F.; Wenzel, P. J. J. Am. Chem. Soc. 1996, 118, 11446. (37) Terrier, F.; Boubaker, T.; Xiao, L.; Farrell, P. G. J. Org. Chem. 1992, 57, 3924.

)

kBH -p [B] p

(3)

(4)

Analysis of the data in Table S4 revealed that only the buffer pathways of eq 3 were important in determining kobsd in the pH range of 5.26-8.81 covered in our experiments.23 In agreement with the reduced eq 4, all plots of kobsd vs free amine concentration ([B]), typically consisting of six to eight points at a given pH, i.e., at a given buffer ratio p ) [B]/[BH], were linear with negligible intercepts. However, a pH dependence of the slopes was observed in buffers with pKCH a - 1 < pH < 23 pKCH a + 1, i.e., the glycine ethyl ester buffers (Figure S3). In B BH these instances, the individual rate constants kp and k-p were determined from a standard treatment of the data obtained at three or more different buffer ratios. In other buffers, eq 4 simplified to either kobsd ) kBp [B] at high pH or kobsd ) BH k-p [BH] at low pH, allowing a facile determination of the kBp BH and k-p rate constants from the slopes of the various kobsd BH vs [B] or [BH] plots. Then, the corresponding k-p or kBp B BH values were calculated by means of eq 5. All kp or k-p rate

kBp

KCH a ) BH BH k-p Ka

(5)

constants are summarized in Table 3. In the case of the 2,4,6trinitrobenzyltriflone 4e (pKCH a ) ) 6.16), a few measurements of the rates of equilibrium attainment according to eq 3 could be made at pH 5.56 and 5.74 in aminoacetonitrile buffers ([B]/ [BH]) ) 2 and 3, respectively). From the variations of the high kobsd values thus obtained with the buffer concentration, an BH rate constant was obtained: estimate of the corresponding k-p BH 4 -1 +1 -1 k-p ≈ 9 × 10 mol L s . Discussion Structures of the Carbanions. Although this approach has been largely used, determining the hybridization for carbon from an analysis of 13C-H coupling constants is now known not to apply satisfactorily well for carbanions.2,38-40 It is therefore through evaluation of the chemical shift variations brought about by the ionization of a carbon acid that reliable information on the structural reorganization which accompanies the formation

R-Sulfonyl Carbanions

J. Am. Chem. Soc., Vol. 120, No. 37, 1998 9499

Table 4. Changes in Chemical Shifts (δ∆HR, δ∆CR) and Coupling Constants (∆1JCRHR) upon Ionization of R-Substituted Toluenes (PhCH2X) in Me2SO compd

X

∆δHRc

∆δCRc

∆Jc

5a 7a 7b 6a 4a 4f 4g

NO2a COCH3a COC6H5a CNa SO2CF3b SO2CH3a SO2C6H5a

0.99 0.76 0.98 -1.64 -1.87 -1.18 -1.14

29.9 40.3 45 11 0.22 4.6 3.6

29.2 23 20.7 12.6 21.1 23 20.7

a

Data taken from ref 2a. b This work. c ∆δ in ppm; ∆J in hertz.

of the conjugate carbanion may be obtained. Such variations (∆δ) are the result of two main opposing effects: (1) a highfield shift caused by the presence of negative charge and (2) a low-field shift caused by a possible sp3 f sp2 rehybridization of the carbanionic center (CR).2,41 Significant upfield shifts of CR and HR resonances are actually the rule in reactions giving rise to sp3-hybridized carbanions, e.g., the formation of alkyllithiums (∆δCR ) -13 ppm for methyllithium).38 In contrast, large downfield shifts in the range 40-60 ppm are typical for reactions where the rehybridization factor is predominant, e.g., the formation of fluorenyllithium in THF (∆δCR ) 43.6)38 and of the fluorenyl “carbanion” derived from (η5-cyclopentadienyl)(η6-fluorenyl)iron(II) cation in Me2SO (∆δCR ) 49; ∆δHR ) 1.7).42 On the other hand, intermediate ∆δ values are found in reactions giving rise to carbanions whose R-carbons are between sp3 and sp2 in hybridization, e.g., ∆δCR ) 15.4 ppm for formation of the unsubstituted benzyl anion.39,43 r-X-Substituted Toluene Anions (PhCHX-). On the basis of the above reasoning, a comparison in Table 4 of the ∆δCR and ∆δHR values associated with the ionization of various R-substituted toluene carbon acids (PhCH2X) is very informative regarding the structure of the resulting carbanions (PhCHX-). As can be seen, large downfield shifts of the CR and HR resonances occur upon ionization of phenylnitromethane (5a; ∆δCR ) 29.90; ∆δHR ) 0.99) and the benzyl ketones 7a (∆δCR ) 40.30; ∆δHR ) 0.76) and 7b (∆δCR ) 45; ∆δHR ) 0.98) in Me2SO.2 Such ∆δ values agree well with the traditional visualization of the related conjugate carbanions as the nitronate or enolate oxyanions C-5a and C-7a,b, respectively.2a,4a In contrast, the ionization of the benzyltriflone 4a induces a very strong upfield shift of the HR resonance: ∆δHR ) -1.87. Concomitantly, the CR resonance moves negligibly to low field: ∆δCR ) 0.22. Despite the much stronger electron-withdrawing effect exerted by the SO2CF3 group (vide infra), the situation closely resembles that observed in the ionization of benzylmethyl (∆δCR ) 4.60; ∆δHR ) -1.18) and benzylphenyl (∆δCR ) 3.60; ∆δHR ) -1.14) sulfones (4f and 4g) as well as of benzylcyanide (6a; (38) Browne, S. E.; Asher, S. E.; Cornwall, E. H.; Frisoli, J. K.; Harris, L. J.; Salot, E. A.; Sauter, E. A.; Trecoske, M. A.; Veale, P. S., Jr. J. Am. Chem. Soc. 1984, 106, 1432 and references therein. (39) Bywater, S.; Lachance, P.; Worsfold, D. J. J. Phys. Chem. 1975, 79, 2148. (40) Takahashi, K.; Kondo, Y.; Asami, R.; Inoue, Y. Org. Magn. Reson. 1974, 6, 580. (41) (a) Stothers, J. P. In Carbon-13 NMR Spectroscopy; Academic Press/ New York, 1972; p 210. (b) Simonnin, M. P.; Xie, H. Q.; Terrier, F.; Lelievre, J.; Farrell, P. G. J. Chem. Soc., Perkin Trans. 2 1989, 1553. (c) Terrier, F.; Vichard, D.; Chatrousse, A. P.; Top, S.; MacGlinchey, M. J. Organometallics 1994, 13, 690. (d) Kovar, K. A.; Breitmaier, E. Chem. Ber. 1978, 111, 1646. (42) Johnson, J. W.; Treichel, P. M. J. Am. Chem. Soc. 1977, 99, 1427. (43) (a) O’Brien, D. H. In ComprehensiVe Carbanion Chemistry; Buncel, E., Durst, E., Eds.; Elsevier: New York, 1980; Vol. 5, Chapter 6 (The Nuclear Magnetic Resonance of Carbanions). (b) Nelson, G. L.; Williams, E. A. Prog. Phys. Org. Chem. 1976, 12, 229.

∆δCR ) 11.00; ∆δHR ) -1.64).2a This suggests that a high negative charge density must be retained on the exocyclic carbon of the triflone carbanion (structure C-4a) and the two related sulfonyl anions (structures C-4f,g), as it does in the case of the

nitrile anion (structure C-6a). It follows that both the resonance contributors C-4S and C-4H which imply significant C ) S double-bond character as well as the aromatic contributor C-4Ar cannot be of appreciable importance in the stabilization of the three R-sulfonyl carbanions. Structures C-6aN and C-6aAr play similarly a minor role in the stabilization of C-6a.2a,3

Ionization of 4a induces only a weak upshield shift of the fluorine resonance (∆δF ) -2.25), supporting the idea that most of the negative charge of C-4a is not dispersed through the SO2CF3 group. On the other hand, the upfield shift of HR increases in the order SO2C6H5 ∼ SO2CH3 < CN < SO2CF3, suggesting that in this series the possibility of having even a minor delocalization of charge through the phenyl ring decreases when the electron-withdrawing character of the exocyclic X group is increased. Interestingly, it has been shown that, with an sp2hybridized carbon and no steric restrictions of resonance, maximum charge delocalization onto the para carbon of a phenyl ring results in a maximum ∆δC4 value of about -25 ppm, while a purely inductive effect by an aromatic ring results in a negative para ∆δ of < 10 ppm.38 In the case of C-4a, a ∆δC4 value of - 12.76 ppm is found (Table S2),23 comparable with that reported for the phenylnitromethane anion C-5a (∆δC4 ) -9.48).2a,44 This similarity is obviously consistent with a minor contribution of the structures C-4Ar to the stabilization of C-4a. For comparison, the upfield shift is ∆δC4 ) -21.6 ppm for formation of the unsubstituted benzyl anion (as a lithium salt) despite the fact that this anion is not completely sp2-hybridized.38 (44) Bradamante, S.; Pagani, G. A. J. Org. Chem. 1980, 45, 105.

9500 J. Am. Chem. Soc., Vol. 120, No. 37, 1998 The 1JCRHR coupling constants for the various PhCH2X and PhCHX- species discussed above are included in Table 4. As can be seen, 1JCRHR is always much higher in the carbanion than in the parent toluene substrate with the ∆J values being rather similar for the two ketone and the three sulfone compounds (∆J ∼ 21-23 Hz). While similar ∆J values for formation of R-sulfonyl and R-sulfoxide carbanions have been previously accounted for in terms of a notable sp2 hybridization of the CR carbon,45 they are here clearly difficult to reconcile with the very different responses of the CR and HR resonances to the ionization of the CH2X group.46 This supports the aforementioned view that analysis of 13C-H coupling constants may not afford clear-cut conclusions regarding hybridization and charge delocalization in carbanions.2,38-40 Ring-Substituted Benzyl Triflones. Table 1 shows that the effect of increasing the efficiency and the number of electronwithdrawing groups in the phenyl ring of the unsubstituted triflone 4a is to reduce progressively the upfield shift of the HR resonance from -1.87 to -0.01 ppm upon ionization. Concomitantly, ∆δCR increases from essentially zero to 24.4 ppm on going from 4a to 4e. These trends suggest a regular increase in the contribution of the activated phenyl ring to charge delocalization (structures C-4Ar1,2,3 in the case of C-4e), with a rather similar situation in C-4d and C-4e, in accord with the idea that there is commonly some steric restriction to resonance due to the o-nitro groups in 2,4,6-trinitrocyclohexadienyl-type carbanions as compared with 2,4-dinitro analogues.41b,47

A most noteworthy feature, however, is that the above highest ∆δHR and ∆δCR values do not compare at all with those associated to the ionization of ring R-substituted toluenes known to afford sp2-hybridized cyclohexadienyl-type carbanions, e.g., ∆δHR ) 1.96-2.31, ∆δCR ) 53.36-57.7 ppm for the conversion of the picryl ketones 8a-d to the anions C-8a-d.21,41d,48 Such values are so much higher than those found for formation (45) (a) Chassaing, G.; Marquet, A.; Corset, J.; Froment, F. J. Organometallic Chem. 1982, 232, 293. (b) Chassaing, G.; Marquet, A. Tetrahedron 1978, 34, 1399. (c) Lett, R.; Chassaing, G.; Marquet, A. J. Organomet. Chem. 1976, 111, C-17. (d) Biellmann, J. F.; Vicens, J. J. Tetrahedron Lett. 1978, 467. (46) A reviewer has suggested that the sets of ∆δHR, ∆δCR, and ∆J values associated with the ionization of 4a, 4f, and 4g may be understood in terms of a rehybridization of the anionic CR atom occurring with a minor charge transfer to the oxygen atoms of the SO2R groups (structure C-4S). Since there is concomitantly convincing NMR evidence that the contribution of the aromatic ring to the charge transfer (structure C-4Ar) is negligible, a strong rehybridization of CR upon ionization of 4a, 4f, and 4g would imply that the negative hyperconjugation mechanism operates to a large extent in the stabilization of the conjugate carbanions C-4a, C-4f, and C-4g (structure C-4H). This situation is not borne out by 19F NMR data since the ionization of 4a does not result in a notable upfield shift of the fluorine resonance. Also, the kinetic and thermodynamic data obtained in H2OMe2SO mixtures do not support this conclusion, emphasizing a rather similar behaviour of benzyl sulfones and benzylcyanides (vide infra). Interestingly no hyperconjugated form analogous to C-4H can be envisioned in the case of R-cyanocarbanions. (47) (a) Lelievre, J.; Farrell, P. G.; Terrier, F. J. Chem. Soc., Perkin Trans. 2 1986, 333. (b) Buncel, E., Venkatachalam, T. K.; Menon, B. C. J. Org. Chem. 1984, 49, 413. (48) That the picryl ketone carbanions exist essentially in the form C-8Ar1 (or its o-nitro analogues) was evidenced in particular by the observation of a nonequivalence of the H3,5 protons as well as of the C2,6 and C3,5 carbons of their picryl ring.21.

Terrier et al.

of C-4e that they provide clear, though indirect, support for a lesser importance of the aromatic resonance contributors in the stabilization of our triflone carbanions than in the related benzyl ketone carbanions. Also consistent with this conclusion is the observation that the visible spectrum of C-4e shows an absorption maximum at a much shorter wavelength (λmax ) 437 nm) than found for typical trinitrotoluene anions, including C-8a-d (λmax > 510 nm).21,41b,49 Then, we are left with the view that the negative charge of the carbanions C-4a-e is essentially retained at the CR carbon, including in the picryl system.50 Equilibrium Acidities of 4a-e. Solvent Dependence of the Acidifying Effect of the SO2CF3 Group. Comparison in Table 5 of the pKa value for ionization of the triflone 4a with those previously reported for related R-substituted toluenes (PhCH2X) shows that the acidifying effect of the SO2CF3 group is considerably larger than that of a carbonyl, a cyano, or a SO2R group and almost as large as that of a NO2 group in Me2SO. This confirms previous observations made by Bordwell et al. in a study of the ionization of methane carbon acids (CH3X) in this solvent.5 That the PhCH2X compounds are all more acidic than the corresponding CH3X compounds mainly reflects the acidifying inductive effect of a phenyl group.55 Examination of the effect of solvent on pKa values is most informative regarding the relative acidifying behavior of the NO2 and SO2CF3 substituents. As can be seen in Table 2, the acidity of the triflones 4a-e tends to decrease while that of the arylnitromethane compounds (5a,c) strongly increases upon addition of increasing amounts of water to a Me2SO solution. As a result, the SO2CF3 group is considerably less acidifying than a NO2 group in aqueous solution as compared to Me2SO. Thus, the acidity difference between 4a and 5a increases from 2.30 pK units to 7.17 pK units on transfer from Me2SO to a 50% H2O-50% Me2SO mixture. Similarly, this solvent change strongly increases the difference in the pKa values of the paranitro-substituted analogues 4c and 5c: ∆pKa ) 0.84 in Me2SO; ∆pKa ) 4.57 in 50% Me2SO. In the case of nitroalkanes and arylnitromethanes, it is well recognized that the solvent dependence of the pKa values reflects the favorable effect of hydrogen-bonding solvation on the (49) All 2,4,6-trinitrotoluene carbanions exhibit rather similar λmax values, presumably because steric restriction to resonance through the o-nitro groups favors the role of the paraquinoid structure in the delocalization of the negative charge, e.g. λmax ) 514 nm for 2,4,6-trinitrotoluene anion.47b (50) An additional feature is worthy of comment. It has been shown experimentally and accounted for by theoretical calculations that delocalization of a negative charge through nitro-substituted phenyl as well as cyclohexadienyl rings results in large upfield shifts of the resonances of the nitro-bearing carbons ortho and/or para to the reaction site.51-54 The corresponding ∆δ (Cortho) and ∆δ (Cpara) values lie commonly in the range -10, -17 ppm and -18, -30 ppm, respectively. As can be deduced from Table S2, it is only for the picryl system that the ∆δ(C2,6) and ∆δ(C4) values pertaining to the ionization of 4a-e approach the lower limit indicated above: ∆δ (C2,6) ) -11.59; ∆δ(C4) ) -18.57 ppm. Again, this result agrees well with the idea of an appreciable but certainly not predominant delocalization of negative charge through the activated phenyl ring of our triflone carbanions.

R-Sulfonyl Carbanions

J. Am. Chem. Soc., Vol. 120, No. 37, 1998 9501

Table 5. Effect of the X Substituent on the Acidity of R-X-Substituted Toluenes and Related Methane Derivatives in Me2SOa X

2X pKPhCH a

3X pKCH a

NO2 SO2CF3 COC6H5 COCH3 CN SO2C6H5 SO2CH3

12.32 14.62 17.70 19.40 21.90 23.40 25.40

17.20 18.76 24.70 26.70 31.20 29.04 31.1

a T ) 25 °C; this work for PhCH2SO2CF3; other pKa values taken from refs 5, 30, 57b, and 59.

stabilization of the conjugate nitronate oxyanions in aqueous solution (structure C-5solv).4,20,28,56-58 Should a canonical structure of this type notably contribute to the stabilization of the carbanions C-4a-e, as earlier suggested by Bordwell,5 the hard negatively charged oxygen atoms of the SO2CF3 group should also be sensitive to hydrogen-bonding solvation, thereby leading to an enhanced acidity of the parent triflones in aqueous solution (structure C-4Ssolv). Obviously the experimental results do not

fit this proposal. Instead, they agree with the idea that most of the negative charge of the triflone carbanions must reside at the CR carbon, at least in the case of the unsubstituted species C-4a. Structure C-4 is much more favorable than structures C-4S or C-4H in terms of polarizability of the negative charge of the carbanion, making it more susceptible to stabilization by Me2SO than by water.57-61 This would account for the enhanced acidity of the parent triflone in the dipolar aprotic solvent. Interestingly, Table 2 also reveals that the stabilizing effect of Me2SO on the carbanions C-4a-e increases slightly, but (51) (a) Olah, G.; Mayr, H. J. Org. Chem. 1976, 41, 3448. (b) Renfrow, R. A.; Strauss, M. J.; Terrier, F. J. Org. Chem. 1980, 45, 471. (c) Simonnin, M. P.; Pouet, M. J.; Terrier, F. J. Org. Chem. 1978, 43, 855. (52) (a)Chudek, J. A.; Foster, R.; Marr, A. W. J. Chem. Soc., Perkin Trans. 2 1987, 1341. (b) Machacek, V.; Sterba, V.; Lycka, A.; Snobl, D. J. Chem. Soc., Perkin Trans. 2 1982, 355. (53) (a) Wennerstro¨m, H.; Wennerstro¨m, O. Acta Chem. Scand. 1972, 26, 2883. (b) Hosoya, H.; Hosoya, S.; Nagakura, S. Theor. Chim. Acta 1968, 12, 117. (54) Terrier, F. In Nucleophilic Aromatic Displacement; Feuer, H., Ed.; VCH Publishers: New York, 1991; pp 93-95. (55) (a) Bordwell, F. G.; Bares, J. E.; Bartmess, J. E.; McCollum, G. J.; Van Der Puy, M.; Vanier, N. R.; Matthews, W. S. J. Org. Chem. 1977, 42, 321. (b) Bordwell, F. G.; Bares, J. E., Bartmess, J. E.; Drucker, G. E.; Gerhold, J.; McCollum, G. J.; Van der Puy, M.; Vanier, N. R.; Matthews, W. S. Ibid. 1977, 42, 326. (56) Bernasconi, C. F.; Bunnell, R. D. J. Am. Chem. Soc. 1988, 110, 2900. (57) (a) Buncel, E.; Wilson. H. AdV. Phys. Org. Chem. 1977, 14, 133 and references therein; (b) Streitwieser, A., Jr.; Juaristi, E.; Nebenzahl, L. L. Stud. Org. Chem. 1980, 5, 323. (58) (a) Bordwell, F. G.; Bartmess, J. E.; Hautala, J. A. J. Org. Chem. 1978, 43, 3107, 3113, 3116. (b) Bordwell, F. G.; Drucker, G. E.; McCollum, G. J. Ibid. 1982, 47, 2504. (c) Bordwell, F. G.; Branca, J. C.; Hughes, D. L.; Olmstead, W. N. Ibid. 1980, 45, 3305. (59) (a) Matthews, W. S.; Bares, J. E.; Bartmess, J. E.; Bordwell, F. G.; Cornforth, F. J.; Drucker, G. E.; Margolin, Z.; McCallum, R. J.; McCollum, G. J.; Vanier, N. R. J. Am. Chem. Soc. 1975, 97, 7006. (b) Bordwell, F. G.; Bartmess, J. E.; Drucker, G. E.; Margolin, Z., Matthews, W. S. Ibid. 1975, 97, 3226. (c) Bordwell, F. G. Pure Appl. Chem. 1977, 49, 963. (60) Parker, A. J. Chem. ReV. 1969, 69, 1. (61) Bernasconi, C. F.; Terrier, F. J. Am. Chem. Soc. 1987, 109, 7115.

regularly, with increasing the activation of the phenyl ring. Again, this finding can be readily reconciled with NMR results which have suggested that the contribution of the phenyl ring to the delocalization of the negative charge (structure C-4Ari) increases to some extent from C-4a to C-4e. Because of this delocalization, the polarizable character of the carbanion and therefore its potential stabilization by Me2SO increases.21,57 In the NMR section, we have emphasized the similar electronic mode of action of the CN and SO2R substituents in the stabilization of R-substituted carbanions. In this regard, it is interesting to note in Table 2 that the effect of adding Me2SO to aqueous solution is to increase the acidity of the five benzyl cyanides 6b-f in the same way at it does for the triflones 4a-e.35,36 In view of the much lower electron-withdrawing effect of a CN group compared to a SO2CF3 group, one might argue that the activating substituents in the aromatic ring induce a much more important transfer of negative charge through the phenyl moiety in the case of R-cyano than R-SO2CF3 carbanions. Thus, the enhancing effect of Me2SO on the acidity of 6b-f might be primarily viewed as the result of stabilization of the conjugate carbanions C-6b-f through resonance structures of type C-6Ar1 or C-6Ar2 rather than of type C-6. However, this

explanation is not very attractive, at least in the case of the mononitro derivatives for the following reasons: (1) the observation that the ionization of 6b results in a notable highfield shift of the HR resonance (∆δHR ) -0.23) leaves little doubt that an appreciable portion of the negative charge resides at the CR carbon in the 4-nitrophenylacetonitrile anion62 and (2) it is well-known that an o-NO2 group is significantly less effective at resonance stabilization of negative charge than is a p-NO2 group.54 Since the para substituents of 6d-f exert only an inductive effect, it seems reasonable to assume that the conjugate carbanions C-6d-f must also be primarily viewed as C-6 rather than C-6Ar2. Our conclusion is therefore that the enhancing effect of Me2SO on the acidity of 6b-f must be, at least in part, a reflection of an increased stabilization of the carbanionic structures C-6 by this dipolar aprotic solvent. Accordingly, the situation for benzylcyanides seems really comparable to that which prevails in the case of benzyltriflones, further favoring the view that both R-cyano and R-SO2R carbanions have a large polarizable negative charge which is mostly localized at the R-carbon. Intrinsic Reactivity of Benzyltriflones 4a-e. Figure 1 shows that a good Bro¨nsted plot may be drawn on the basis of the measured rate constants kBp pertaining to the deprotonation of the 2,4-dinitrobenzyltriflone 4d by primary amines in 50% Me2SO. The corresponding βB value is equal to 0.38, being very close to the range of 0.50 ( 0.10 commonly found for ionization of carbon acids of similar pKCH values in this a solvent.4,21,28,35-37,56,61,63 Using the classical definition of the BH intrinsic rate constant, i.e., k0 ) (kBp /q) when pKCH a - pKa (62) (a) Crampton, M. R. J. Chem. Soc. (B) 1967, 85. (b) Minch, M. J.; Giaccio, M.; Woeff, R. J. Am. Chem. Soc. 1975, 97, 3766. (c) Walters, E. A. J. Phys. Chem. 1977, 81, 1995.

9502 J. Am. Chem. Soc., Vol. 120, No. 37, 1998

Terrier et al. Table 6. Comparison of Intrinsic Rate Constants for Deprotonation of Some Representative Carbon Acids by Primary and Secondary Amines in 50% H2O-50% Me2SOa

Figure 1. Bro¨nsted plot for ionization of 4d by primary amine buffers in 50% H2O-50% Me2SO (v/v): T ) 25 °C, I ) 0.5 M (NMe4Cl). The numbering of the various catalysts is given in Table 3. 2 log (p/q) ) 0, the log kRNH value for the ionization of 4d was 0 readily determined from Figure 1: log k0 ) 3.46.4,20,56,64 Using BH BH rate constant (k-p ) 9 × 104 mol-1 L+1 s-1) the k-p determined in aminoacetonitrile buffers and assuming RBH ) 2 for the picryltriflone 4e could 0.50, a rough value of log kRNH 0 also be estimated by means of eq 6: log k0 ) 4.50 ( 0.30. On the other hand, the fact that the rates of deprotonation of 4a-c CH by base reagents of pKBH a ’s close to the related pKa ’s are not accessible by stopped-flow spectrophotometry means that the intrinsic reactivity of these derivatives are higher than that of 4e, i.e., log k0 g 5.

KCH a log k0 ) log kBH + R log -p BH BH Ka

(6)

In the past decade, it has been recognized that there is commonly an inverse relationship between the intrinsic reactivity of a carbon acid and the extent of the structural and solvational reorganization which is required to form the conjugate carbanion.4,20 The greater the resonance stabilization and therefore the sp2-hybridized character of the resulting carbanion, the greater, in general, are the structural and solvation changes involved in the ionization process and the lower the intrinsic reactivity.4,20,21,35-37,56,58,63 Table 6 reveals a first significant result, 2 values measuring the intrinsic namely that the log kRNH 0 reactivity of 4a-e are much higher than those associated with the ionization of carbon acids giving rise to strongly resonancestabilized sp2-hybridized nitronate and enolate carbanions. Representative examples of this category quoted in Table 6 include nitroalkanes such as phenylnitromethane 5a, polynitrotoluene derivatives such as 2,4,4′-trinitrodiphenylmethane, or the picryl ketones 8a-d as well as β-dicarbonyl compounds such as acetylacetone and 1,3-indandione. In these instances, low to very low log k0 values are obtained because an enormous structural reorganization is required to form the conjugate bases, e.g., see structures C-5a and C-8Ar.4,41b,63,65 Here, it is of particular interest to note that the intrinsic reactivities of the triflones 4a-e are about 2 orders of magnitude higher than those (63) (a) Bernasconi, C. F.; Paschalis, P. J. Am. Chem. Soc. 1986, 108, 2969. (b) Bernasconi, C. F.; Bunnell, R. D.; Terrier, F. Ibid. 1988, 110, 6514. (c) Bernasconi, C. F.; Ohlberg, D. A. A.; Stronach, M. W. J. Org. Chem. 1991, 56, 3016. (64) (a) Marcus, R. A. J. Phys. Chem. 1968, 72, 891. (b) Cohen, A. O.; Marcus, R. A. Ibid. 1968, 72, 4249. (65) Terrier, F.; Lelievre, J.; Chatrousse, A. P.; Farrell, P. G. J. Chem. Soc., Perkin Trans. 2 1985, 1479.

CH acid

2 log kRNH 0

RCH(CN)2 4a-c 4e 9-cyanofluorene 4d 6b 1,3-indandione 6c acetylacetone 8a CH3NO2 2,4,4′-trinitrodiphenylmethane C6H5CH2NO2 (5a)

∼7b g5c ∼4.5c 3.76d 3.46c 3e 2.44f 2.05e 2.06f 0.85g 0h -0.65i -1h

log kRR′NH 0

4.58d 3.70e 3.13f 2.75e 2.75f 0.73h -0.10i -0.25h

a At 25 °C unless indicated otherwise; log kRNH2 and log kRR′NH 0 0 refer to primary and secondary amines as deprotonating reagents, respectively. b Reference 4. c This work. d Reference 61 at 20 °C. e References 35 and 36 at 20 °C. f Reference 63. g Reference 21a. h Reference 28 at 20 °C. i Reference 65.

of ketone carbon acids. Interestingly, Bunting and Wodzinski have found a similar difference in the reactivities of a number of structurally related sulfones and ketones but the exact log k0 values could not be determined.66 For a more complete understanding of the behavior of 4ae, it is once again interesting to compare the situation with that previously found for the four nitriles listed in Table 6. Thus, aliphatic nitriles, such as those of general structure RCH(CN)2, have high intrinsic reactivities because they afford carbanions whose stabilization derives essentially from the polar effect of the cyano group.4,35,36 Introduction of a R group such as a phenyl or a fluorenyl moiety provides a notable potentiality of resonance stabilization of the carbanion, as reflected by a decrease in log 2 kRNH from about 7 to 3.76 for 9-cyanofluorene.61 Introduction 0 of nitro groups in the aromatic moiety further decreases the 2 value of 3 measured intrinsic reactivity. Thus, the log kRNH 0 for the ionization of 4-nitrophenylacetonitrile 6b is understandable only if the nitrosubstituted ring contributes appreciably to the resonance stabilization of the carbanion C-6b (structure C-6Ar1), even though it is also evident from NMR and pKa results that a notable charge density remains located on CR (vide supra).35 On the other hand, the log k0 value of 2.05 measured for 2,4-dinitrophenylacetonitrile 6c is sufficiently low to suggest that delocalization through the activated ring must be in this instance the main factor contributing to the stabilization of the carbanion C-6c.35 Going now to the triflone series, we note that 2 value, i.e., the 2,4the compound with the lowest log kRNH 0 dinitro compound 4d (log k0 ) 3.46), exhibits an intrinsic reactivity which is not only 30-fold higher than that measured for the cyanide analogue 6c but also 3-fold greater than that for the mononitrosubstituted cyanide 6b. More importantly, the four other triflones have log k0 values which are located at a relatively high level, i.e., log k0 g 4-5, in the intrinsic reactivity scale.67 It seems to us that this situation is in itself most convincing evidence for a weak occurrence of charge delocalization and hence, a large predominance of the pyramidal carbanionic structure C-4 rather than of the sp2-hybridized (66) (a) Stefanidis, D.; Bunting, J. W. J. Am. Chem. Soc. 1991, 113, 991. (b) Stefanidis, D.; Bunting, J. W. Ibid. 1990, 112, 3163. (c) Wodzinski, S.; Bunting, J. W. Ibid. 1994, 116, 6910. (67) As suggested by a reviewer, the greater log k0 value for 4e than for 4d is consistent with the evidence that the presence of the two o-NO2 groups results in a notable steric inhibition to resonance in 2,4,6-trinitrotoluene carbanions.4,41b,47

R-Sulfonyl Carbanions canonical representations C-4S, C-4H, and C-4Ar1 in the stabilization of the R-SO2CF3 carbanions C-4a-e. Conclusion The three different approaches used in this work to study the ionization of the triflones 4a-e lead to a unique structural picture of the resulting carbanions C-4a-e. On the basis of in particular our finding of high intrinsic reactivities for these compounds, it appears clearly that stabilization of a negative charge by the strong electron-withdrawing SO2CF3 group must be the result of polarization effects rather than of conjugative d-p π-bonding or negative hyperconjugation, at least in solution.68 Interestingly, this situation is not fully in agreement with conclusions reached from high-level ab initio studies or crystal structure analyses.7,17-19,22,23 These favor the view that R-sulfonyl carbanions with a CR carbon bearing at least one aryl group are planar and that hypernegative conjugation operates significantly together with polarization effects in determining the stability of these species, especially in the case of SO2CF3 derivatives.17-19 As pointed out by Streitwieser et al. in a recent study of the equilibrium acidities of some sulfones and sulfoxides in THF,70 solid-state and computed gas-phase structures may not always be a perfect guide for structures and reactions in solution. This is apparently the case in the systems described in this work. Experimental Section Materials. Solvents were purified and solutions made up as described previously.37,65 Buffers were purified commercial products. The synthesis of the various triflones 2a-e, either by Hendrickson’s method71a or a new methodology, was recently reported elsewhere.71b Measurements. pH of buffer solutions in H2O-Me2SO mixtures [I ) 0.5 M (NMe4Cl)] were measured potentiometrically as described before.37,65 In these solvents, the spectrophotometric determination of the pKCH values of 4a-e was facilitated by the fact that the anions a C-4a-e exhibit a strong UV-visible absorption in regions of the (68) In response to a reviewer’s comment, our results do not allow us to discriminate between polarizability and inductive/field effects, as has been done for various systems.69. (69) Bernasconi, C. F.; Kittredge, K. W. J. Org. Chem. 1998, 63, 1944. (70) Streitwieser, A.; Wang, G. P.; Bors, D. A. Tetrahedron 1997, 53, 10103. (71) (a)Hendrickson, J. B.; Giga, A.; Wareing, J. J. Am. Chem. Soc. 1974, 96, 2275. (b) Goumont, R.; Faucher, N.; Moutiers, G.; Tordeux, M.; Wakselman, C. Synthesis 1997, 691.

J. Am. Chem. Soc., Vol. 120, No. 37, 1998 9503 spectrum where neither the parent triflones nor the required buffers absorb. Spectra were taken on a KONTRON-UVICON-930 spectrophotometer; λmax values of C-2a-e were quoted in the Results. The same spectrophotometric procedure was applied, using amine buffers, to measure the pKCH a values of 4b-e in pure Me2SO. In the 2SO case of the much less acidic unsubstituted triflone 4a, the pKMe was a determined with reference to two indicators i.e., 2,4-dinitroaniline and 4,4′-dinitrophenylmethane, for measuring the pH of buffer solutions made up from this triflone with [C-4a]/[4a] ratios equal to 1/3, 1/2, 1/1, 2/1, and 3/1. These solutions were prepared by addition of potassium dimsyl in Me2SO so that the molarity of the anionic species C-4a was in all cases equal to 0.01 M. Under these experimental conditions, the pKCH value of 4a was calculated from eq 2 (see a Results). Kinetic measurements of the ionization of 4d,e were carried out at 25 ( 0.2 °C with an Applied Photophysics stopped-flow spectrophotometer. NMR spectra were recorded on a Brucker AC300 spectrometer. Chemical shifts are reported in ppm (J values in hertz) with tetramethylsilane (1H, 13C) and CFCl3 (19F) as the internal references. The various triflones were dissolved in Me2SO-d6 (∼0.2 M), and the carbanions were generated in solution by adding an equimolar amount of methanolic potassium methoxide (∼5.5 M) to the above solutions. Under these experimental conditions, the stability of these species was sufficient as to provide not only well-resolved 1H, 19F, and noise protondecoupled 13C spectra but also proton-coupled 13C spectra (Tables S1S3).

Acknowledgment. This work enters in part in the frame of the European TMR Program ERB FMRX-CT97-0120 entitled “Fluorine as a unique tool for engineering molecular properties”. Supporting Information Available: Figure S1 (variation of the ratio of ionized to un-ionized benzyltriflones 4 as a function of pH in 50% H2O-50% Me2SO (v/v); I ) 0.5 M (NMe4Cl)), Figure S2 (ionization of 2,4-dinitroaniline in various benzyltriflone buffers in Me2SO), Figure S3 (effect of pH and buffer concentration on the observed rate constant, kobsd, for deprotonation of 2,4-dinitrobenzyltriflone 4d by primary amine buffers in 50% H2O-50% Me2SO (v/v); T ) 25 °C, I ) 0.5 M (NMe4Cl)), Table S1 (1H and 19F NMR data for benzyltriflones 4a-e and related carbanions C-4a-e), Tables S2 and S3 (13C NMR data for benzyltriflones 4a-e and related carbanions C-4a-e), and Table S4 (kinetic data; kobsd values) (9 pages, print/PDF). See any current masthead page for ordering information and Web access instructions. JA9806074